The present invention relates to a method for wet etching of a thin film for flattening a thin film formed on a substrate such as a semiconductor wafer.
In the manufacture of a semiconductor device, a thin film of silicon oxide, silicon nitride, etc. is formed on a semiconductor wafer as an insulating film. A batch type chemical vapor deposition (CVD) method is widely employed for forming such a thin film.
In a silicon oxide film formed by a batch type CVD method, however, the thickness of the film tends to be larger in the peripheral portion of the wafer than in the central portion of the wafer. In the batch type CVD method, a process gas is supplied from a peripheral portion toward a central portion of the wafer. As a result, the process gas concentration is high in the peripheral portion. Also, the film-forming temperature is high in the vicinity of a heater. It follows that the silicon oxide film tends to be thickened in the peripheral portion of the wafer, as pointed out above.
If a silicon oxide film having a nonuniform thickness is etched for forming contact holes, the contact hole formed through a peripheral portion having a larger thickness tends to have a smaller diameter. On the contrary, the contact hole formed through a central portion having a smaller thickness tends to have a larger diameter. The nonuniformity in diameters of the contact holes gives rise to a low yield of the product semiconductor device.
A thin film formed by another film-forming method also tends to be nonuniform in thickness in some cases. For example, a thin film formed by a one-by-one type CVD method tends to be thicker in a central portion than in a peripheral portion of the wafer. The nonuniformity in the diameters of the contact holes formed through the film also takes place in this case, leading to a low yield of the product semiconductor device.